Method of Manufacturing Light Emitting Diodes and Light Emitting Diode

ABSTRACT

In an embodiment a light emitting diode includes an n-type n-layer, a p-type p-layer and an intermediate active zone configured to generate ultraviolet radiation, a p-type semiconductor contact layer having a varying thickness and a plurality of thickness maxima directly located on the p-layer and an ohmic-conductive electrode layer directly located on the semiconductor contact layer, wherein the n-layer and the active zone are each of AlGaN and the p-layer is of AlGaN or InGaN, wherein the semiconductor contact layer is a highly doped GaN layer, and wherein the thickness maxima have an area concentration of at least 104 cm−2.

This is a divisional application of U.S. application Ser. No.16/493,499, entitled “Method of Manufacturing Light Emitting Diodes andLight Emitting Diode,” which was filed on Sep. 12, 2019, which is anational phase filing under section 371 of PCT/EP2018/056158, filed Mar.13, 2018, which claims the priority of German patent application102017105397.2, filed Mar. 14, 2017, all of which are incorporatedherein by reference in its entirety.

TECHNICAL FIELD

A method for the manufacture of light emitting diodes is specified. Inaddition, a light emitting diode is specified.

SUMMARY OF THE INVENTION

Embodiments provide a light emitting diode which has a high lightcoupling-out efficiency in the ultraviolet spectral range.

According to at least one embodiment, light emitting diodes are producedby the method. The finished light emitting diodes emit incoherentradiation during operation, i.e., no laser radiation. In particular,they are light emitting diodes used to generate ultraviolet radiation,for example, with a wavelength of maximum intensity in the spectralrange between 200 nm and 400 nm or including 200 nm and 300 nm.

According to at least one embodiment, the method comprises the step ofgrowing an n-conductive n-layer. The n-layer may be formed by a singlelayer or composed of several sublayers, each of which is n-doped, forexample, with silicon. Optionally, the n-layer contains a thin sublayerwhich is not necessarily n-doped and which may serve as a barrier layerfor positive charge carriers, for example, with a thickness of at most15 nm or 10 nm.

According to at least one embodiment, an active zone is grown. Theactive zone is configured to generate ultraviolet radiation. Thepreferred wavelength of maximum intensity of the radiation generated bythe active zone in the finished light emitting diode is at least 205 nmor 217 nm and/or 360 nm or 310 nm or 270 nm or 230 nm. The preferredwavelength of maximum intensity is between 205 nm and 260 nm. The activezone preferably contains a multiple quantum well structure, also knownas MQW.

According to at least one embodiment, a p-conductive p-layer is grown.The p-layer may contain several sublayers, each of which is preferablyp-doped, for example, with magnesium. Optionally, the p-layer contains asublayer that is not necessarily doped as a barrier layer for negativecharge carriers, i.e., for electrons, with a thickness of at most 15 nmor 10 nm. Preferably the active zone on opposite sides directly adjoinsthe n-layer and the p-layer.

According to at least one embodiment, the method comprises the step ofproducing a p-type semiconductor contact layer. The semiconductorcontact layer is part of the semiconductor layer sequence. Thesemiconductor contact layer is preferably heavily p-doped. Furthermore,the semiconductor contact layer is in direct contact with the p-layer atleast in places. This means that the semiconductor contact layer and thep-layer touch each other. Current injection into the p-layer ispreferably carried out exclusively or predominantly, at least 90% or95%, via the semiconductor contact layer.

According to at least one embodiment, an electrode layer is applied tothe semiconductor contact layer, in particular directly to thesemiconductor contact layer. The electrode layer can also be composed ofseveral sublayers. The electrode layer preferably contains one or moremetal layers. The electrode layer is ohmic-conductive.

According to at least one embodiment, the electrode layer is applieddirectly to or on the semiconductor contact layer, so that the electrodelayer and the semiconductor contact layer touch each other. It ispossible that the electrode layer touches the p-layer, where a geometriccontact area between the electrode layer and the p-layer can be largerthan between the electrode layer and the semiconductor contact layer.For example, there is no or no significant current flow from theelectrode layer directly into the p-layer, so that current is fed fromthe electrode layer into the semiconductor contact layer and from thereinto the p-layer. The electrode layer is especially configured as ananode. The contact layer is particularly necessary because theelectrical barrier between the electrode and the p-layer iscomparatively high for physical reasons. During operation, a currentflow directly into the p-layer is therefore desired, but less orprevented than via the contact layer.

According to at least one embodiment, the n-layer and the active zoneare each based on AlGaN. The n-layer and the active zone are preferablyfree of indium and each has, at least in places, a high aluminumcontent.

According to at least one embodiment, the p-layer is made of AlGaN or ofInGaN or of AlInGaN.

According to at least one embodiment, the semiconductor contact layer isa GaN layer. This means that the semiconductor contact layer ispreferably free of indium and aluminum.

In terms of the material compositions of the respective layers, only theessential components of the crystal lattice, i.e., aluminum, gallium,indium and nitrogen, are mentioned above. The respective layers maycontain additional components in small quantities, such as oxygen,carbon, silicon and/or magnesium. Such impurities and/or doping shallpreferably not exceed 0.1% by weight and/or at most 10²³/cm³ or10²¹/cm³.

According to at least one embodiment, the semiconductor contact layerhas a varying thickness. The semiconductor contact layer has a largenumber of thickness maxima. The thickness maxima are separated from eachother by areas with a smaller thickness of the semiconductor contactlayer. Lower thickness in this context includes the case that thesemiconductor contact layer may have a thickness of zero in places. Thismeans that the thickness maxima can be formed by island-shaped,non-contiguous regions of the semiconductor contact layer.

According to at least one embodiment, the thickness maxima have an areaconcentration of at least 10⁴/cm² or 10⁵/cm² or 10⁶/cm². Alternativelyor additionally, this area concentration is at most 10⁹/cm² or 10⁸/cm²or 10⁷/cm². In other words, many of the thickness maxima are present inthe finished light emitting diodes. For example, the finished lightemitting diodes have an average edge length of at least 0.25 mm or 0.5mm and/or not more than 1.5 mm or 1 mm.

According to at least one embodiment, the thickness maxima aredistributed irregularly. This means that, seen from above, the thicknessmaxima do not represent a regular grid. In particular, the thicknessmaxima are evenly distributed over the p-layer within the borders ofstatistical fluctuations, so that no areas with intentionally high orintentionally low surface concentrations are formed.

In at least one embodiment, the method is configured for the manufactureof light emitting diodes and comprises the following steps, preferablyin the order indicated:

-   -   A) Growing an n-conducting n-layer,    -   B) Growing an active zone to generate ultraviolet radiation,    -   C) Growing of a p-conductive p-layer,    -   D) Forming a p-type semiconductor contact layer having a varying        thickness and having a plurality of thickness maxima directly        on, at or in the p-layer; and    -   E) Applying an ohmic-conducting electrode layer directly on the        semiconductor contact layer, wherein the n-layer and the active        zone are each based on AlGaN and the p-layer is based on AlGaN        or InGaN and the semiconductor contact layer is a GaN layer and        the thickness maxima have an area concentration of at least 10⁴        cm⁻² or 10⁶ cm⁻² seen in top view.

Due to the material properties of AlGaN with a high aluminum content, inparticular due to the very high activation energy of the usual acceptormagnesium, even compared to GaN, the production of p-doped AlGaN layerswith a high aluminum content of sufficient conductivity and sufficientlylow contact resistance for light emitting diodes, or LEDs for short, inparticular for the generation of ultraviolet radiation, is only possibleto a very limited extent. For this reason, UV LEDs usually use thin,continuous p-GaN layers on a p-side. Since the emission wavelength ofthese light emitting diodes is below the absorption edge of p-GaN ofabout 360 nm, such a continuous p-GaN layer absorbs a significantportion of the generated radiation and does not emit it.

With the light emitting diodes described here, three-dimensionalstructures, i.e., small islands containing p-GaN, are formed as anelectrical connection layer during growth on the p side, which otherwiseconsists mainly of AlGaN. A metallic contact, for example, is thenapplied as an electrode layer that electrically connects the p-GaNislands and injects current there. Instead of metallic contacts,contacts made of transparent conductive oxides, TCOs for short, may alsobe used to form the electrode layer.

Due to the significantly reduced volume of the strongly absorbing p-GaNfor the semiconductor contact layer the extraction probability forultraviolet radiation is significantly increased. Such an increase can,with a high reflectivity of the electrode layer, be more than a factorof 2. Thus, the external quantum efficiency can be significantlyincreased. The electrical efficiency of light emitting diodes, on theother hand, is usually reduced by the reduced p contact area caused bythe structuring of the semiconductor contact layer. In view of thesignificantly increased extraction probability, however, the effect iscompensated by the reduced p contact area, so that the light emittingdiode is more efficient overall.

According to at least one embodiment, the p-layer is only partiallycovered by the semiconductor contact layer when viewed from above. Thedegree of coverage of the p-layer by the semiconductor contact layer ispreferably at least 0.1% or 0.5% or 2% and/or at most 20% or 15% or 10%or 5%. Thus, the external quantum efficiency can be increased with highreflectivity of the electrode layer.

According to at least one embodiment, a side of the electrode layerfacing the semiconductor contact layer and/or a side of the active zonefacing away from the p-layer are planar. In particular, this means thatthe aforementioned sides are free from intentionally producedstructuring. Any unevenness is thus due to undesirable defects. Planarmeans, for example, a mean roughness of 2 nm or 0.5 nm or 0.25 nm.

According to at least one embodiment, the semiconductor contact layer isformed by a plurality of contact islands. Thus, it is possible that thesemiconductor contact layer is not a continuous layer. In particular,there is no connection between the contact islands made of a material ofthe semiconductor contact layer itself.

According to at least one embodiment, adjacent thickness maxima have amean distance from each other of at least 0.1 μm or 0.4 μm or 1 μm or 3μm when viewed from above. Alternatively or additionally, the meandistance shall not exceed 100 μm or 30 μm or 10 μm. If the semiconductorcontact layer shows areas which have a maximum thickness over a largerarea and thus an areal thickness maximum, then a point lying in themiddle of this area, seen from above, is preferably regarded asthickness maximum. In this case, this means that per island-shaped areaof the semiconductor contact layer there is exactly one point-shapedthickness maximum in top view.

The thickness maxima preferably correspond to one of the contactislands, so that there can be an unambiguous assignment between thecontact islands and the thickness maxima and/or the contact islands areidentical with the thickness maxima. The above values for the thicknessmaxima therefore apply in the same way to the contact islands.

According to at least one embodiment, there is a continuous sublayer onone side of the semiconductor contact layer facing the electrode layer.The sublayer is a part of the semiconductor contact layer and ispreferably made of the same material as the thickness maxima. Theelectrode layer is preferably in direct contact with this sublayer overthe entire surface. The sublayer may connect all thickness maxima witheach other.

According to at least one embodiment, the sublayer of the semiconductorcontact layer is thin. In particular, a thickness of the sublayer shallbe at least 2 nm or 5 nm or 10 nm and/or at most 50 nm or 30 nm or 20nm.

According to at least one embodiment, the thickness of the sublayer issmaller than an average height of the thickness maxima. In particular,the sublayer is at least 1.5 or 3 or 5 times thinner than the averagethickness of the thickness maxima.

According to at least one embodiment, the thickness maxima are formed byV-defects filled with a material of the semiconductor contact layer. TheV-defects are caused by the targeted opening of defects such asdislocations in the semiconductor layer sequence. The opening of thedefects, so that the V-defects are pronounced, takes place, for example,in the p-layer, close to an interface to the active zone. Alternatively,the defects can already be opened in the active zone or in the n-layerto the V-defects. The resulting V-defects are preferably completelyfilled by the material of the semiconductor contact layer. Preferably,the opened V-defects are limited to the p-layer. This means that thepreviously linear, tubular defects such as dislocations are specificallyopened and widened funnel-shaped or in the form of inverted pyramids.

According to at least one embodiment, the V-defects have an openingangle of at least 20° or 30° or 40° after opening. Alternatively oradditionally, this opening angle shall not exceed 110° or 90° or 75°. Inparticular, the opening angle is approximately 60°.

According to at least one embodiment, the semiconductor layer sequence,in particular the p-layer, comprises an opening layer. In or at theopening layer, the previously existing defects, in particular thedislocations, are opened to the V-defects.

According to at least one embodiment, the opening layer is made ofAlInGaN or InGaN. In particular, the opening layer contains indium. Anindium content of the opening layer is preferably not more than 20% or10% or 5%. For example, the thickness of the opening layer shall be atleast 5 nm or 10 nm or 15 nm and/or at most 50 nm or 35 nm or 20 nm. Ifan opening layer is present, all remaining areas of the p-layer arepreferably made of AlGaN, so that the opening layer can be the only partof the p-layer containing indium.

According to at least one embodiment, the opening layer is located onone side of the p-layer facing the active zone. The opening layer canform a boundary between the active zone and the p-layer or be locatedwithin the p-layer. A distance between the active zone and the openinglayer is preferably not more than 50 nm or 30 nm or 15 nm.

According to at least one embodiment, in a step C₁) between steps C) andD), a masking layer is produced on a side of the p-layer facing awayfrom the active zone. The masking layer is preferably located directlyon the p-layer. The masking layer is preferably made of a dielectricmaterial, especially a nitride such as silicon nitride. A thickness ofthe masking layer is small, for example, at most 3 nm or 2 nm or 1 nm.In particular, the masking layer shall have an average thickness of notmore than 1.5 monolayers or 1 monolayer.

According to at least one embodiment, the finished masking layer onlypartially covers the p-layer. The masking layer preferably has a largenumber of openings. In particular, the openings are statisticallydistributed across the p-layer. The masking layer can be formedself-organized, e.g., determined by the duration of a supply of startingmaterials for the masking layer. This means that it is not necessary forthe masking layer to be structured using lithography or a stampingmethod, for example.

According to at least one embodiment, the semiconductor contact layer isformed in the openings, starting from the p-layer. This means that thesemiconductor contact layer is grown on the p-layer from the openings.The material of the semiconductor contact layer preferably does not growon the mask layer itself.

According to at least one embodiment, exactly one contact island of thesemiconductor contact layer is generated per opening of the mask layer.Neighboring contact islands are preferably not interconnected by amaterial of the semiconductor contact layer, i.e., in particular byp-doped GaN. The masking layer is preferably only partially covered bythe material of the semiconductor contact layer. Due to the growth ofthe contact islands, the material of the semiconductor contact layer canextend onto the masking layer at one edge of the openings when viewedfrom above.

According to at least one embodiment, the thickness maxima, inparticular the contact islands and/or the filled V-defects, show p-dopedGaN. A dopant concentration is preferably at least 10¹⁹ cm⁻³ or 10²⁰cm⁻³ and/or at most 10²³ cm⁻³ or 10²² cm⁻³. A dopant is preferablymagnesium.

According to at least one embodiment, the contact islands and/or theV-defects are covered with an intermediate layer in a step D₁) betweensteps D) and E). The intermediate layer may be made of a semiconductormaterial such as undoped or doped AlGaN. Alternatively, it is possiblethat the interlayer is made of a dielectric material with a lowrefractive index compared to the semiconductor layer sequence, forexample, an oxide such as silicon dioxide or a nitride such as siliconnitride. An intermediate layer composed of sublayers, such as asemiconductor material and a dielectric material, is also possible. Viaa dielectric layer of the intermediate layer, in particular directly atthe masking layer and/or at the p-layer, a high reflectivity ofultraviolet radiation can be achieved via total reflection over asubstantial range of angles of incidence.

According to at least one embodiment, the contact islands and/or theV-defects are planarized with the intermediate layer. A continuous,contiguous layer is preferably formed by the intermediate layer togetherwith the contact islands and/or the V-defects.

According to at least one embodiment, the intermediate layer after stepD₁) completely or partially covers the contact islands.

According to at least embodiment, the method comprises a step D₂)performed between steps D₁) and E). In step D₂), the intermediate layeris partially removed so that the contact islands and/or the V-defectsfilled with the material of the semiconductor contact layer are exposed.This enables efficient contacting of the semiconductor contact layerwith the electrode layer.

According to at least one embodiment, the individual layers of thesemiconductor layer sequence, in particular the n-layer, the activezone, the p-layer and/or the semiconductor contact layer, are producedby means of metal organic vapor phase epitaxy, MOVPE for short. A growthtemperature for the p-layer is preferably between 700° C. and 1100° C.inclusive, especially between 700° C. and 800° C. inclusive. In otherwords, the p-layer may be grown at comparatively low temperatures,especially in the case of opening V-defects.

Alternatively or in addition, the growth temperature of thesemiconductor contact layer is comparatively high and is at least 900°C. or 950° C. or 970° C. and/or at most 1150° C. or 1100° C. or 1050° C.

In addition, a light emitting diode is indicated. The light emittingdiode is preferably manufactured using a method as indicated inconjunction with one or more of the above embodiments. Characteristicsof the method are therefore also disclosed for the light emitting diodeand vice versa.

In at least one embodiment, the light emitting diode is configured togenerate ultraviolet radiation and comprises an n-conducting n-layer, ap-conducting p-layer and an active zone for generating ultravioletradiation arranged between them. A p-type semiconductor contact layerhas a varying thickness with a variety of thickness maxima with a higharea concentration and is located directly on the p-layer. Anohmic-conductive electrode layer is applied directly to thesemiconductor contact layer.

According to at least one embodiment, the semiconductor contact layer isformed by V-defects filled with highly doped GaN.

According to at least one embodiment, the semiconductor contact layer isformed by contact islands of highly doped GaN. The contact islands startfrom apertures in a masking layer on the p-layer and partially cover themasking layer.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, a method described here and a light emitting diodedescribed here are explained in more detail with reference to thedrawing on the basis of exemplary embodiments. Same reference signsindicate the same elements in the individual figures. However, there areno references to scale shown, rather individual elements may beexaggeratedly large for a better understanding.

In the Figures:

FIGS. 1A and 1B show schematic sections of variations of light emittingdiodes;

FIGS. 2A, 3A, 5A, 5B, 6A, 7A and 7C show schematic sectional views ofexemplary embodiments of method steps of a method;

FIGS. 2B, 3B, 6B and 7B show schematic top views of exemplaryembodiments of method steps of a method; and

FIG. 4 shows a schematic cross-sectional view of an exemplary embodimentof a light emitting diode.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

In FIG. 1 a modification of a light emitting diode is shown. The lightemitting diode is configured for emission of ultraviolet radiation R. Ona growth substrate 51, the light emitting diode comprises asemiconductor layer sequence 2, see FIG. 1A. The semiconductor layersequence 2 comprises a buffer layer 21, an n-conductive n-layer 22, anactive zone 23, a p-conductive p-layer 24 and a continuous, arealp-conductive semiconductor contact layer 25. An ohmic-conductiveelectrode layer 3 is located on the semiconductor contact layer 25.

Radiation R generated in the active zone 23 is usually reflected severaltimes in the semiconductor layer sequence 2 before the radiation R iscoupled out from a roughening 26, see FIG. 1B. Since the semiconductorcontact layer 25 absorbs the radiation R, comparatively large absorptionlosses occur at the areal semiconductor contact layer 25. This reducesefficiency. A wavelength of the radiation R is below 360 nm,corresponding to the absorption edge of GaN.

In FIGS. 2 and 3 the method steps for the production of light emittingdiodes described here are illustrated, figure parts A showing asectional view and figure parts B showing a schematic top view.

According to FIG. 2A, a growth substrate 51 is used, such as aluminumnitride. Buffer layer 21, for example, is made of aluminum nitride orAlGaN and is optionally n-doped. The n-layer 22 is an n-doped AlGaNlayer. The active zone 23 is based on AlGaN and contains non-drawnquantum well layers as well as few drawn barrier layers. The active zone23 is followed by the p-layer 24 over the entire surface, which is madeof p-doped AlGaN, with an aluminum content, for example, of between 10%and 90% inclusive.

V-Defects 41 is opened at a border region of p-layer 24 to the activezone 23. Seen in cross-section the V-defects 41 are triangular, seen intop view they are hexagonal, see FIG. 2B. The V-defects 41 show inparticular the shape of inverted, mostly regular hexagonal pyramids. Inorder to achieve opening of the V-defects 41 on previously existing,non-drawn dislocations, the p-layer 24 is particularly preferred grownat a combination of comparatively low temperatures, in particularT≤1000° C., comparatively high pressure, in particular p≥200 mbar,comparatively high hydrogen content in the gas phase and/orcomparatively weakly doped with magnesium, for example, with a dopantconcentration of ≤2×10¹⁹ cm⁻³. An opening angle of the V-defects 41,seen in cross-section, is approximately 60°.

FIG. 3 illustrates that the V-defects 41 are filled with a material forthe semiconductor contact layer 25, wherein reference numeral 4 denotesa thickness maxima. The semiconductor contact layer 25 is of p-GaN witha dopant concentration of preferably at least 10¹⁹ cm⁻³ to 10²¹ cm⁻³,whereby magnesium is used as the dopant. The semiconductor contact layer25 is preferably deposited at a combination of comparatively hightemperatures, in particular T≥900° C., and comparatively highly dopedwith magnesium, for example, with a dopant concentration of ≥2*10¹⁹cm⁻³.

The semiconductor contact layer 25 in the V-defects 41, for example,extends at least 50% or 75% and/or at most 90% or 95% through thep-layer 24. Deviating from the representation in FIGS. 2 and 3, theV-defects can also begin in the n-layer 22 or in the active zone 23 (notshown). A thickness of p-layer 24, for example, is, as is preferred inall other exemplary embodiments, at least 50 nm or 100 nm or 200 nmand/or at most 500 nm or 300 nm. Seen from above, the semiconductorcontact layer 25 only covers a small part of the surface of the p-layer24.

Optionally, a sublayer 43 of the semiconductor contact layer 25 can beproduced over the entire surface of the filled V-defects 41 and on thep-layer 24. This sublayer 43 forms an improved contact with theelectrode layer 3, which is subsequently applied, e.g., of aluminum. Thesublayer 43 has only a small thickness, for example, between 5 nm and 15nm, so that the absorption of UV radiation in the sublayer 43 is weak.

The method steps of FIGS. 2 and 3 are preferably carried out in a wafercomposite. Steps such as applying a carrier substrate, detaching thegrowth substrate 51 or singulation into individual light emittingdiodes, and applying electrical contact structures are each not shown tosimplify the representation.

In FIG. 4 an exemplary embodiment of the LED 10 is shown. The carriersubstrate 52 is used for electrical contacting via electrode layer 3.Current flows dominantly from electrode layer 3 into semiconductorcontact layer 25 and from this into p-layer 24 and over this into theactive zone 23. Reference numeral 4 denotes a thickness maxima. Then-side 22, for example, can be electrically connected via an electricalcontact surface 55, such as a bond pad. As in all other exemplaryembodiments, the growth substrate 51 is optionally removed and aroughening 26 is provided to improve the light extraction.

Due to the triangular cross-sectional area of the filled V-defects 41, alarge ratio of the outer surface of the semiconductor contact layer 25to its volume is realized. Thus, on the one hand an efficient currentinjection into the p-side 24 can be achieved, and on the other hand, dueto the small volume, only little absorption of UV radiation takes placein the semiconductor contact layer 25. By exploiting defects such asdislocations and the targeted opening of the V-defects, small structuresand small mean distances between adjacent thickness maxima 4 can beachieved. Since the V-defects 41 are completely filled, a smooth surfacecan be achieved on one side of the p-layer 24 and the semiconductorcontact layer 25 facing away from the active zone 23, whereby anincreased reflectivity can be achieved on the electrode layer 3, whichis preferably a mirror.

In FIG. 5, the semiconductor layer sequence 2 additionally has anopening layer 44. The opening layer 44 is, for example, made of AlInGaN,with a comparatively low indium content. This makes it possible to openthe V-defects 41 in or at the opening layer 44 in a targeted manner.

According to FIG. 5A, the opening layer 44 is located at the boundarybetween the active zone 23 and the p-layer 24, whereas according to FIG.5B, the opening layer 44 is located within the p-layer 24, for example,with a distance to the active zone 23 of about 10 nm.

In FIGS. 6 and 7 a further example of a manufacturing method for theproduction of light emitting diodes is shown. Deviating from FIG. 2, noV-defects are generated in the p-layer. The prevention of the opening ofV-defects is achieved in particular by growing the p-layer 24 atcomparatively high temperatures, especially at 1200° C.+/−50° C., incontrast to FIG. 2. A masking layer 45 is applied locally to the p-layer24, for example, of silicon nitride and with an average thickness ofonly less than one monolayer up to several monolayers. The masking layer45 contains a large number of openings which can be self-organized andin which the p-layer 24 remains free, see FIG. 6B.

According to FIG. 7A, the material for the semiconductor contact layer25 is deposited starting from the openings in the masking layer 45, sothat thickness maxima 4 are formed in the form of individual,non-contiguous contact islands 42, see the top view in FIG. 7B.

Optionally, an intermediate layer 46 can be produced before theelectrode layer 3 is applied, which can be done by sputtering or vapordeposition, for example. The intermediate layer 46 serves to planarizethe thickness maxima 4. For example, the intermediate layer 46 is madeof AlGaN, as is the p-layer 24. In this case, the intermediate layer 46can be doped or undoped. Alternatively, a dielectric material such assilicon dioxide is deposited in order to achieve increased reflectivityin the interaction of the electrode layer 3.

According to FIG. 7A, the intermediate layer 46 is optionally depositedin such a way that the contact islands 42 are at least partially coveredby the material of the intermediate layer 46. FIG. 7C illustrates thatafter generating the intermediate layer 46, it is partially removedagain, so that the intermediate layer 46 together with the contactislands 42 form a smooth side facing away from the active zone 23 onwhich the electrode layer 3 is directly applied. Thus, the contactislands 42 are located directly on the electrode layer 3 on the sidefacing away from the active zone 23.

The masking layer 45 is completely covered by the intermediate layer 46together with the contact islands 42. The contact islands 42 at one edgeof the openings partially cover the masking layer 45.

The intermediate layer 46 can be removed mechanically and/or chemically.In contrast to the illustration in FIGS. 7A and 7C, the intermediatelayer 46 may also be composed of several sublayers, in particular toachieve increased reflectivity at the intermediate layer 46 and at theelectrode layer 3.

The proportion of the surface of the p-layer directly covered by thecontact islands 42 can be adjusted by such a masking layer 45, e.g., bythe growth time of the masking layer 45. Instead of a self-organizedmasking layer 45, structuring can alternatively be carried out using astamping method or lithographic methods.

As an alternative to a metallic, reflecting electrode layer 3, anelectrode layer transparent to the generated radiation R can be used ineach case, in particular made of transparent conductive oxides, TCOs forshort. For example, Ga2O₃, ITO or a Sr—Cu oxide, individually or incombination, can be used.

Creating the intermediate layer 46 is optional. If the intermediatelayer 46 is omitted, electrode layer 3 is applied directly to themasking layer 45 and the contact islands 42 and has a comparativelyrough, structured side facing the active zone 23. This is particularlypossible if the electrode layer 3 is formed from a TCO.

Light emitting diodes 10 described here are used, for example, for gassensors to detect certain gas absorption lines. For example, awavelength of maximum intensity of the generated radiation R liesbetween 217 nm and 230 nm. An emitted radiant power of the radiation R,for example, is approximately 1 mW.

Unless otherwise indicated, the components shown in the figures followeach other directly in the order indicated. Layers not touching eachother in the figures are spaced from each other. As far as lines aredrawn parallel to each other, corresponding surfaces are also parallelto each other. Also, unless otherwise indicated, the relative thicknessratios, length ratios and positions of the drawn components to eachother are correctly reproduced in the figures.

The invention described here is not limited by the description given byway of the exemplary embodiments. Rather, the invention includes eachnew feature as well as each combination of features, which in particularincludes each combination of features in the patent claims, even if thatfeature or combination itself is not explicitly stated in the patentclaims or exemplary embodiments.

1. A light emitting diode comprising: an n-type n-layer, a p-typep-layer and an intermediate active zone configured to generateultraviolet radiation; a p-type semiconductor contact layer having avarying thickness and a plurality of thickness maxima directly locatedon the p-layer; and an ohmic-conductive electrode layer directly locatedon the semiconductor contact layer, wherein the n-layer and the activezone are each of AlGaN and the p-layer is of AlGaN or InGaN, wherein thesemiconductor contact layer is a highly doped GaN layer, and wherein thethickness maxima have an area concentration of at least 10⁴ cm⁻².
 2. Thelight emitting diode according to claim 1, wherein the semiconductorcontact layer is formed by V-defects filled with highly doped GaN. 3.The light emitting diode according to claim 1, wherein the semiconductorcontact layer is formed by contact islands of highly doped GaN.
 4. Thelight emitting diode according to claim 3, wherein a masking layer isarranged on a side of the p-layer facing away from the active zone,wherein the masking layer only partially covers the p-layer and has aplurality of openings, and wherein the semiconductor contact layer isarranged in the openings of the masking layer starting from the p-layerso that the contact islands are formed.
 5. The light emitting diodeaccording to claim 3, wherein the contact islands, which are of GaN withan Mg dopant concentration between 10¹⁹ cm⁻³ and 10²³ cm⁻³, inclusive,are planarized with an intermediate layer so that a continuouscontiguous layer is formed by the intermediate layer and the contactislands.
 6. The light emitting diode according to claim 5, wherein theintermediate layer and the contact islands directly adjoin the electrodelayer.
 7. The light emitting diode according to claim 6, wherein theintermediate layer and the contact islands terminate flush at a sidefacing the electrode layer.
 8. The light emitting diode according toclaim 5, wherein the intermediate layer is of undoped AlGaN.
 9. Thelight emitting diode according to claim 1, wherein a side of theelectrode layer facing the semiconductor contact layer and a side of theactive zone facing away from the p-layer are planar.
 10. The lightemitting diode according to claim 1, wherein the semiconductor contactlayer is formed by a plurality of contact islands and is not acontinuous layer.
 11. The light emitting diode according to claim 10,wherein a degree of coverage of the p-layer by the semiconductor contactlayer is between 0.5% and 10%, inclusive, and wherein adjacent thicknessmaxima, which are equal to the contact islands, have, seen in top view,an average distance from one another between 1 μm and 30 μm, inclusive.12. The light emitting diode according to claim 1, wherein the thicknessmaxima are formed by V-defects filled with material of the semiconductorcontact layer, and wherein the V-defects have an opening angle between30° and 90°, inclusive.
 13. The light emitting diode according to claim12, wherein the p-layer comprises an opening layer in or at which thedefects open to the V-defects, and wherein the opening layer is ofAlInGaN or InGaN and contains indium and all remaining regions of thep-layer are of AlGaN.
 14. The light emitting diode according to claim13, wherein the opening layer is located on a side of the p-layer facingthe active zone and a distance between the active zone and the openinglayer is at most 30 nm.
 15. The light emitting diode according to claim14, wherein exactly one contact island is present per opening of amasking layer and adjacent contact islands are not interconnected by amaterial of the semiconductor contact layer itself so that the maskinglayer is only partially covered by the material of the semiconductorcontact layer.
 16. The light emitting diode according to claim 1,wherein the electrode layer comprises a transparent conductive oxidedirectly on the semiconductor contact layer, or the electrode layerconsists of at least one transparent conductive oxide.
 17. The lightemitting diode according to claim 1, wherein the ultraviolet radiationhas a wavelength of maximum intensity between 205 nm and 260 nm,inclusive.